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Old 13 June 2019, 22:38   #1
klx300r
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Question A4000 ATX convertor cable not working with my ATX PSU??

tried https://amigakit.amiga.store/product...roducts_id=615 with on new micro-ATX PSU and an older standard size one and I can't get my A4000 to boot at all....after slight heart attack from thinking I messed up my A4000 I put my original PSU back in and thank Amiga goodness she boots up just fine SO:


1- Chris from Amigakit says it's due to some newer PSU's shutting down if they don't detect 'sense' voltage to stop PSU operating at no current draw?


2- how can I get around this issue WITHOUT buying an 'older' PSU
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Old 13 June 2019, 23:18   #2
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Yep, that happens. It's not that new a phenomenon either, as any decent ATX PSU should do the same if the voltages go out of spec. The problem is showing up more recently because PSUs expect heavier power usage with modern PCs, and because the bulk of a PSU's output has shifted to the 12V rail. But it's a good thing that it shuts down - if it didn't, there's a risk that the voltages might wander too low (causing crashing) or too high (causing damage to the chips). This is sometimes the case with very cheap PSUs because they don't keep check on themselves.

What happens is this: The PSU will usually have a main output, and it will constantly monitor that output to make sure it's the correct voltage. It does this using a sense wire, a smaller wire that runs to the ATX connector and shares one pin with a heavier, current-carrying wire for the primary output. If the output's not correct, it will try to adjust it until it is. If it can't adjust far enough, it should shut down.

Due to the design of the PSUs, they're built to expect a certain minimum load on the main output rail, usually 1A or so, but it can vary, and is rarely published in a PSU spec sheet because any modern PC will easily use far more than that. The regulation circuitry is built with that in mind, and if the minimum load isn't present, it can be impossible to get the output to stabilise at the correct voltage, resulting in the PSU shutting down almost immediately.

All other outputs are based on that primary output, so are all adjusted in proportion with that. Now, back in the olden days, 5V used to be king for digital electronics, so computers used more of that than anything else, and 5V was the main output rail. If this was still the case, you'd be fine because the Amiga will generally take over 1A on the 5V rail. But in the early ATX era, 3.3V became more popular for powering the CPUs, RAM and GPUs of the time. Because of this, the 3.3V rail became the main output, and stayed like that for many years. The Amiga doesn't use any 3.3V at all, so there's never any load on that output. Sometimes this wasn't a problem, because internally the 3.3V and the 5V rails came from the same section of the PSU and therefore either one would do. In more recent times however, the 3.3V rail has reached the limit of how much it can reliably provide through the PCB, and in order to transfer more power to today's monstrous GPUs and CPUs, the voltage needed to be increased. So the 12V rail is now the primary output on most PSUs, and local voltage converters use this to generate the 3.3V, 2.4V or whatever the GPU needs, at a much higher current than the motherboard could transmit. The Amiga does use 12V, but only a very tiny amount since most of its parts are 5V. It might use 0.1A or even less, whereas the PSU is expecting to use at the very least 1A, and probably more than 3A. So again, the PSU can't regulate, and shuts down when it can't get itself in spec.

You can usually tell which rail is the important one by looking at the ATX connector and seeing which pin has a second, thinner wire attached. Many (but not all) follow a colour convention, where orange wires are 3.3V, red are 5V and yellow are 12V. Some PSUs might even have multiple sense wires, effectively being separate, independent supplies.

Now, how to get around this. Well, there isn't really a trick - you just need to give the PSU what it wants - the correct load. If it's a 12V main supply, you'll need to use some power from the 12V rail. Adding a bunch of fans might be enough, but is a noisy solution. A heavy power rating resistor of an appropriate value is a more elegant solution, which will essentially turn a few watts of 12V power into heat. On the 12V rail, a 12 ohm resistor will pass 1A. It will also generate 12W of heat, so you'll need a heatsink, or to attach it to the chassis, and it should be rated for probably 20W to be safe.

Similarly, a resistor can be used to load the 3.3V line if needed - 3.3 ohms will again pass 1A and generate 3.3W of heat so it should be rated at least 5W.

If you're not confident with that sort of solution, I would suggest finding someone who is or else trying a different PSU. What might help is going for the lowest power PSU you can find. In PC terms, usually more watts capacity is better, but they often require a higher minimum load than lower capacity PSUs. There are 300W SFX PSUs available that might be worth checking out - the Amiga will happily run on less than 100W, just make sure the 5V rail supplies maybe 5A and you should be fine.
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Old 13 June 2019, 23:54   #3
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@ Daedalus


thanks for the very detailed explanation Rob The resistor to load the 3.3V line sounds simple to me will also check out the 300W SFX PSUs you mentioend as I might use this nice little new micro-ATX PSU to build a small form PC for my daughter that started high school this year
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